1. Field of the Invention
This invention relates to an optically active compound useful in the fields of liquid crystal display elements or liquid crystal light switching elements, and a liquid crystal composition containing the same. More particularly it relates to a novel liquid crystalline compound having optically active groups and a liquid crystal composition containing the same, and exhibiting a chiral nematic phase or chiral smectic phase.
In addition, the liquid crystalline compound referred to herein is not limited only to compounds having a liquid crystal state which can be observed by themselves, but also it includes substances which, even when no liquid crystal state can be observed by themselves, have a chemical structure similar to that of the former and also are useful as an additive.
2. Description of the Related Art
It is well known that the importance of optically active substances in liquid crystal compositions used for electrooptical elements utilizing liquid crystal phases, i.e. liquid crystal elements or liquid crystal light switching elements, consists in the usefulness of constituting components of
(i) cholesteric phase (chiral nematic phase) and PA0 (ii) chiral smectic phase.
It is also well known that the cholesteric phase (Ch phase) may be composed of as main components, compounds which exhibit Ch phase by themselves, but also may be composed by adding optically active substances to nematic phase and in the latter case, it is unnecessary that the added optically active substances exhibit liquid crystal phases by themselves. It is also known that Ch phase is utilized for display elements utilizing cholesteric-nematic phase transition, and besides, Ch phase is also utilized in place of nematic phase in order to prevent the so-called reverse domain from occurring. Further, optically active substances have been used as a constituting component of cholesteric liquid crystal compositions used for STN mode (a mode having the twist angle of liquid crystals enlarged up to 180.degree.-270.degree.) which is one of relatively new display modes; thus the importance of optically active substances has been more and more increasing.
On the other hand, an electrooptical element using chiral smectic liquid crystals has recently been noted. This element utilizes ferroelectricity of liquid crystals and this novel display mode has a possibility of notably improving the response rate (Clark et al, Applied Phys. lett., 36, 899 (1980)). This display mode is directed to a method utilizing chiral smectic phases exhibiting ferroelectricity such as chiral smectic C phase (hereinafter abbreviated to SC* phase). Ferroelectricity-exhibiting phases are not limited to SC* phase, but chiral smectic F, G, H, I and the like phases have been known to also exhibit ferroelectricity. When these ferroelectric liquid crystals are used for display elements, liquid crystal materials exhibiting ferroelectric liquid crystal phases within a broad temperature region including room temperature have been desired. At present, however, no single compound which satisfies such a requirement has been known; thus liquid crystal compositions which satisfy such required characteristics as much as possible have been prepared and used.
Chiral smectic liquid crystal compositions may also be composed of, as their main component, compounds which exhibit a chiral smectic phase by themselves, as in the case of the Ch phase, but the composition may also be composed by adding optically active substances to smectic phase and in this case, too, it is unnecessary that the added optically active substances exhibit liquid crystal phases by themselves.